The invention generally relates to sealing devices for use in subsea wells.
After a wellbore (in a land well or a subsea well) has been drilled, various operations are performed. Such operations may include logging, perforating, and other operations. In a typical land well, the wellhead equipment includes a lubricator that allows tool strings to be lowered into the wellbore. At the top of the lubricator may be a "stuffing box," which includes a sealing element that seals on the line carrying the tool string as the tool string is run into the well. The line carrying the tool string may be a wireline, a slickline, or a tubing. By sealing on the line, wellbore fluids are prevented from escaping through the wellhead equipment as the tool string is run into the well.
In a subsea well, a blow-out preventer (BOP) is typically located at the subsea well surface (generally referred to as the mud line). Wellbore equipment extends below the BOP into the subsea wellbore. A marine riser extends from the BOP to a sea surface vessel or platform. The marine riser includes a large tubing that isolates fluids in the marine riser from the sea water. Typically, control lines may be run on the outside of the marine riser to the surface vessel or platform. Such control lines may include fluid communication lines (e.g., hydraulic lines or gas pressure lines) and electrical lines. Thus, using the control lines, various types of fluids may be communicated to the BOP and equipment in the wellbore.
In performing logging or perforating operations in a subsea well, the inner bore of the marine riser in many instances is exposed to the wellbore of the subsea well. As logging or perforating tool strings are lowered through the BOP into the subsea wellbore, a sealing mechanism is typically not provided at the mud line during run-in. As a result, limitations are imposed on the types of operations that can be performed. For example, it may be desired to log in the subsea wellbore at an elevated pressure. However, because the marine riser is exposed to the wellbore fluid pressure, such elevated pressure may cause damage to the marine riser. Another example includes overbalanced perforation operations, where the wellbore pressure is raised to a level higher than the pressure of the target formation. In addition, sudden rises in wellbore pressure may occur during perforation operations. Because the marine riser is typically formed of relatively thin-walled tubing to reduce cost and weight of the marine riser, the marine riser may not be able to handle pressures above a certain level.
A need thus exists for a sealing mechanism provided at the mud line of a subsea wellbore during certain types of operations, such as logging or perforating operations.